Helium mass spectrometer leak detection is a well-known leak detection technique. Helium is used as a tracer gas which passes through the smallest of leaks in a sealed test piece. The helium is then drawn into a leak detection instrument and is measured. The quantity of helium corresponds to the leak rate. An important component of the instrument is a mass spectrometer which detects and measures the helium. The input gas is ionized and mass analyzed by the spectrometer in order to separate the helium component, which is then measured. In one approach, the interior of a test piece is coupled to the test port of the leak detector. Helium is sprayed on to the exterior of the test piece, is drawn inside through a leak and is measured by the leak detector.
A schematic diagram of a conventional mass spectrometer is shown in FIG. 1. A dipole magnet 10 produces a magnetic field perpendicular to the plane of FIG. 1. An ion source 12 located between the polepieces of the dipole magnet 10 includes a plate 14 having an entrance slit 16. The ion sources in prior art spectrometers utilize a very small entrance slit of width SE, typically 0.5 millimeter in width and several millimeters in length, to physically define an ion optical object. Ions diverging from this ion optical object are reconverged to an image after some deflection angle in the magnetic field. Bending angles of 90° and 180° are the most common. If the ions diverge from the entrance slit 16 with a half-angle α and travel in a uniform magnetic field on a circular trajectory with radius R, a 1:1 image of the entrance slit is formed after a 180° deflection. This image is broadened by an amount Rα2 which results from the different trajectories in the magnetic field of ions diverging either side of the central trajectory, so that the overall image width is SE+Rα2. A plate 18 defining an exit slit 20 at the image position allows only ions of a given mass-to-charge ratio to pass through to a detector (not shown), which produces an ion current in response to sensed ions. Ions of different mass-to-charge ratio (m/z) travel on different radii and cannot pass through the exit slit 20.
The 180° deflection design has been considered convenient for an inexpensive mass-produced system because the focal positions are thought to be well known. A corresponding tradeoff, however, is the fact the entire ion source, including electron-generating filaments, must be placed inside the magnet gap. The length of the entrance slit is thus reduced by the space taken up by filaments and other ion source components to a fraction of the magnet gap, and the detected signal is thereby reduced. A more serious deficiency of the typical mass spectrometer design is that in simple designs without any ion-optical focusing elements between the ion source and the entrance slit, the entrance slit does not serve as an ideal ion optical object. Ions are formed in the ion source some distance back from the entrance slit and are accelerated toward the slit. The ion optical behavior of the ion source causes the ions to appear to diverge from a point near the source, so that the 180° focal point does not correspond to the position of the exit slit, if this slit is positioned at 180° from the entrance slit, but instead occurs some distance before the exit slit position. In this case, the ion beam broadens again before reaching the exit slit and the detected signal is further reduced or a larger exit slit is required, which reduces the mass resolving power. In such a case, the entrance slit actually functions as an angle-limiting slit.
The ion current in a helium mass spectrometer for very low leak rates is on the order femtoamps. With present state of the art leak detector spectrometer technology, this small signal is difficult to detect with sufficient stability to provide an unambiguous leak rate signal. In order to reach the high sensitivity required in modern leak detector applications, for example 1e-11 std-cc/sec or less, current leak detector spectrometers utilize some type of signal amplification, such as an electron multiplier. Electron multipliers are expensive and complicated components. They require a high voltage power supply, typically at 500 to 1800 volts, and frequent tuning is required. The gain of the electron multiplier decays from the time the unit is switched on. In addition, electron multipliers have a limited lifetime, after which they must be replaced at high cost to the user. Lifetime is particularly short, and gain drift is particularly noticeable, in the typically poor vacuum in which leak detectors usually operate.
Accordingly, there is a need for improved mass spectrometers and methods for trace gas leak detection.